Gloss is an important quality criterion e.g. for assessing the quality of paints, coatings, plastic surfaces and the like. Measuring gloss with results that are repeatable and decisive is, however, exceptionally difficult. From a physical point of view gloss can be defined as the property of a surface in its ability to reflect light. But the human eye fails to "see" this physical aspect, instead there are considerable physiological deviations which make it difficult to standardize the results of gloss measurements.
In Germany there is a draft proposal for an industrial standard--DIN 67 350--which dates back to November 1980 and in which a reflectance method for measuring gloss is proposed as a standard. In this method light from a light source is projected on the surface to be measured and introduced from this surface via a lens into a photocell. Since even slight deviations in the position of the optical components result in a considerable change of the light incident on the photocell, the angle of aperture of the diaphragms used and the alignment of the individual optical components must be exceptionally precise.
Known devices working according to this method feature an aluminum carrier for the optical components. These devices are calibrated by measuring a standard surface having a very high characteristic gloss value, i.e. a high-gloss surface and so-called light trap. A light trap is a surface having no reflectance whatsoever and thus has a gloss value of zero. In the known measurement device it is assumed that the characteristic gloss value, contrary to the physiological considerations, has a linear profile between the value 0 and a high-gloss value. To minimize deviations due to lack of exactness in the geometry of the optical components, there; is the possibility in these known devices of, for instance, distorting the aluminum carrier of the optical components to precisely align the optical components.
A similar method of measuring surface gloss is proposed as a standard also from the International Standard ISO 2813/1978 dating back to the year 1978. In the USA the measurement of gloss is standardized by the ASTM D 523 standard.
DE-OS 34 13 838 discloses an instrument for measuring gloss in which a plurality of sets of optical means is arranged within the instrument so that the reflectance can be measured at differing angles of incidence. For this purpose measuring detectors are provided which receive the reflected light and the electrical output signals of which are evaluated by calculating means. To enable this to be done, a reference value is held in the memory of the device for each set of optical means, this reference value corresponding to the signal as measured at a characteristic gloss value of 100. The intermediate values are then obtained by division.
Practical experience with known devices for measuring gloss indicate that it would be highly desirable to improve the accuracy of these instruments. In addition, it is also known that it is very difficult to achieve the necessary accuracy when adjusting by means of distorting the carrier.